1. Field of the Invention
This invention relates to a power transmission apparatus. More particularly, the invention is suitably used while being assembled in a compressor of a car air conditioner and operated from an external power source, such as an engine, through a belt.
2. Description of the Related Art
A refrigerant compressor of a car air conditioner is driven from an external power source such as an engine through a belt and a pulley. To cut off connection between the engine and the compressor, an electromagnetic clutch may be interposed between them. However, the electromagnetic clutch is not interposed in many cases because the production cost can be decreased when an electromagnetic clutch is not disposed. In this case, a torque limiter (power cutoff member) is disposed in the power transmission apparatus of the compressor for the car air conditioner operated through the belt to avoid disadvantage such as belt damage when the compressor seizes.
The torque limiter uses screw meshing for a part of a power transmission route and utilizes an excessive axial force that occurs at the screw meshing portion, owing to the excessive torque when the compressor seizes (refer to Japanese Unexamined Patent Publication No. 2003-206950, for example). Because this torque limiter system utilizes friction for cutoff, however, it involves the problem that the operation torque of the torque limiter changes with the passage of time because the coefficient of friction changes due to corrosion of the friction surface, as a contact surface, and the adhesion of grease.
In the power transmission apparatuses of the prior art such as the one described above, another torque limiter is known with sealing the friction surface and which avoids the problem described above. The power transmission apparatus of this type has a structure in which a part of the power transmission portion has screw coupling. The torque limiter system utilizing this screw coupling breaks a part of the power transmission route and cuts off the power transmission route by utilizing an excessive axial force occurring at the screw coupling portion by the excessive torque that occurs when the compressor seizes.
The requirement for reducing the power losses of the compressor and the power transmission apparatus are high at present. A technology of reducing a diameter of a shaft is known to reduce the loss of a shaft seal device and a bearing as a sliding loss of a shaft of a compressor. In the power transmission apparatus of the prior art having the torque limiter described above, the reduction of the diameter of a rotary shaft on the output side invites another problem. When the diameter of the rotary shaft is decreased in the fastening structure between a hub and the compressor of the power transmission apparatus of the prior art for transmitting power to the compressor, the problem occurs in that the torque limiter mechanism does not operate.
FIG. 5 is a partial sectional side view of an embodiment of the fastening structure of the prior art. In the case of the power transmission apparatus 50 for fastening the rotary shaft 5 of the compressor having a slide portion 5d with the shaft seal device having a relatively large outer diameter, seat faces (bearing surfaces) 8a and 8b of a washer 8 sufficient for power transmission can be arranged. The power cutoff member 3 is fastened at its screw portion 3d to the rotary shaft 5 and the hub 2 mechanically fitted is coupled or meshed with the power cutoff member 3. It can be appreciated from FIG. 5 in this construction that a support surface having a sufficient area for the axial force generated as the torque transmitted from the pulley is converted can be formed on the seat faces 8a and 8b of the washer 8 and on the seat face (bearing surface) 5a of the rotary shaft 5. Therefore, the surface pressure acting on the seat faces 8a and 8b and the seat face 5a can be limited to a low level.
FIG. 6 shows the power transmission apparatus 60 when the fastening structure shown in FIG. 5 is extended to a rotary shaft having a small diameter. FIG. 7 is an enlarged view of the fastening portion in FIG. 6. The area of the seat face 5a of the rotary shaft 5 becomes small, the axial force supporting area of the seat faces 8a and 8b (particularly 8b) of the washer 8 becomes small, and a high torque occurs in the screw fastening direction in the screw portion 3d of the power cutoff member 3 and in the screw portion 5b of the rotary shaft 5 during the high torque operation due to the high load operation of the compressor, so that the seat face 5a of the rotary shaft 5 and the seat faces 8a and 8b of the washer 8 undergoes plastic deformation and the continuous operation cannot be made. Also, the seat face 5a of the rotary shaft 5 and the seat faces 8a and 8b of the washer 8 undergo deformation due to the high torque resulting from the seizure of the compressor and the notch portion 3c provided to the power cutoff member 3 cannot cut-off power. Incidentally, reference numerals of the constituent portions of the prior art examples shown in FIGS. 5 to 7 correspond to the reference numerals of similar constituent portions in the embodiment shown in FIGS. 1 and 2.
Another prior art technology that makes portions in the proximity of the rotary shaft compact in the power transmission apparatus for the compressor is known (Japanese Unexamined Patent Publication No. 2001-173759, for example) but the reference does not disclose the present invention.